Composite electrode for storage batteries and the like

ABSTRACT

A metal/synthetic-resin composite electrode for a storage battery or the like comprises a perforate metal support which is flanked on both sides by openworks of polyolefin, e.g. partially fluorinated polyolefin, which are welded together within the openings of the support and carry the active material. The polyolefin openworks, together with the active material, are covered in turn with inorganic-fiber fabric or felt permeable fine porous cover layers. The inorganic fiber, although preferably glass, can also be mineral wool or asbestos fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.885,184 filed Mar. 10, 1978 now U.S. Pat. No. 4,161,569 and which, inturn, is related to the presently copending application Ser. No. 885,180of Mar. 10, 1978. The present application is also related to theconcurrently filed copending application Ser. No. 39,766. All of theabove applications are in the name of the present inventor.

FIELD OF THE INVENTION

The present invention relates to a so-called composite electrode forstorage batteries and other systems in which an electrochemically activemass must be provided upon a conductive support and, more particularly,to an improvement over the electrode described in application Ser. No.885,184.

BACKGROUND OF THE INVENTION

It is known to provide electromechanical systems which are reversible,such systems including, for example, storage batteries havingrechargeable electrodes. In, for example, lead-acid batteries and otheraccumulators, it is a common practice to apply the active mass (e.g.lead/lead oxide mass) to a metal carrier which serves as a mechanicalsupport for the active mass and as a current collector or a currentdistributor upon discharge/charge cycling.

Because of the dependency of the life of the battery on retention of themass on the support, considerable research has been carried out ortechniques for improving the mechanical stability of the electrode uponcharge/discharge cycling.

For example, the electrode may have one or more synthetic-resin layerswhich form pockets receiving the active mass. Pocketed metallic grids,wooden frame members and like systems have also been used to preventmigration of the active mass from and along the support.

The term "composite electrode" has been used by many to refer tometal/synthetic-resin electrode systems and reference may be had in thisconnection to the German published application (Auslegeschrift) No.1,231,326 and U.S. Pat. No. 3,060,254 which deal with compositeelectrodes.

While such composite electrodes have been successful in large measure,because they do reduce the instability of the electrode structure, itcannot be entirely precluded that the active mass will, as a result ofshape change during the charge/discharge cycling and especially as aresult of swelling and contraction of the electrode, shed the activemass.

In prior-art systems, therefore, there is at least a partial loss ofactive material as a result of charge/discharge cycling.

The active material appears to deposit outside the synthetic-resin layeror to fall out of the pockets formed thereby. The synthetic-resin layersthemselves are usually produced by the sintering of synthetic-resinpowders or from synthetic-resin fibers.

To avoid these problems it has also been proposed to use tubularconstruction for the electrode, these electrodes being referred togenerally as sheathed electrodes. In electrodes of the latter type, theswelling pressures, which are a consequence of volume changes in theactive mass, are taken up by the tubes from which the electrodes areconstituted. Within the tubes there are provided metallic wires servingas current collectors of conductors. The prime difficulty with suchelectrodes is that they are expensive to fabricate and frequently cannotbe made in an entirely reproducible manner so that all of the electrodesgive the same mechanical and electrical characteristics.

As noted previously, considerable effort has been expended in developingtechniques in electrode fabrication and electrode structures so that adetailed review of the art, in this connection, would be impossible topresent.

However, reference may be had, in this connection, to the followingnonexclusive list of patents which may be considered pertinent art:

British Pat. No. 1,018,971

U.S. Pat. No. 482,043

U.S. Pat. No. 1,051,147

U.S. Pat. No. 1,158,491

U.S. Pat. No. 2,515,204

U.S. Pat. No. 2,858,352

U.S. Pat. No. 3,560,262

U.S. Pat. No. 3,772,089

U.S. Pat. No. 3,890,160

U.S. Pat. No. 3,973,991

U.S. Pat. No. 4,048,406

U.S. Pat. No. 4,055,711

U.S. Pat. No. 4,090,897

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide animproved composite electrode which overcomes disadvantages of earliersystems and which extends the principles set forth in my copendingapplication Ser. No. 885,184 mentioned previously.

It is another object of this invention to provide ametal/synthetic-resin composite electrode which does not deteriorate asa result of volume changes in the active mass during charge/dischargecycling and which can be fabricated in a particularly simple and highlyreproducible manner.

SUMMARY OF THE INVENTION

As noted, this invention is an extension of the principles set forth inSer. No. 885,184.

In that application, I have disclosed a metal/synthetic-resin compositeelectrode for storage batteries, accumulators and the like in which ametal current collector and support is provided in the form of aperforated generally flat body with throughgoing openings through whichaccess can be afforded from one side of the electrode to the other. Thissupport is covered on both sides or flanks with the synthetic-resinopenwork layers which receive the active mass. To prevent shape changeor, at least, limit it to the extent that it is not detrimental or asignificant problem, these synthetic-resin openworks are formed as nets,screens, fabrics or grids of synthetic-resin filaments or strands andare fused together through openings in the support, advantageously sothat the fusion point lies wholly within the body of the support.

In a preferred embodiment, the synthetic-resin openworks which receivethe active mass are overlain by synthetic-resin cover layers offine-porous construction which are fused to the synthetic-resinopenworks. These fine-porous synthetic-resin layers can form a sheath inwhich the active mass is enclosed.

In carrying out the fabrication of a composite electrode according tothe invention, mechanical pressure and ultrasonic energy is applied tolocally fuse the two synthetic-resin openworks together within theopening of the support while pressure and heat is applied to bond thefine-porous cover layers to the synthetic-resin openworks.

The resulting structures have been found to have especially long lives,to be highly reproducible and, in general, to be a significant advanceover the art of composite electrodes.

I have now discovered that even this improvement can be markedlyadvanced, quite surprisingly, by the use of selected synthetic-resinmaterials for the openwork and inorganic-fiber fine-porous cover layers.

While one would normally expect that any synthetic-resin materialsresistant to the electrolyte may be used for the openwork andfine-porous cover layers, this has not proved to be the case and, inparticular, I have found that critical to the life (number ofcharge/discharge cycles) of the electrode is the choice of thesynthetic-resin material from which the openwork layer is composed andthe material from which the porous sheath layer is constituted.

More particularly, I have found that a marked increase in the usefullife of the composite electrode can be achieved when the openwork isconstituted of fibers, filaments or strands of polyolefin, especially apartially fluorinated polyolefin and if simultaneously the cover layersare constituted by an inorganic-fiber fabric or felt, preferably aglass-fiber or felt.

The "felt" of the present invention is a nonwoven mat-like orfleece-like structure which can be needled to increase the coherency.

In a preferred embodiment of the invention, the synthetic-resin openworkdesigned to be fused through the openings of the support and to carrythe active mass, is composed of a low-homologous polyolefin or apartially fluorinated polyolefin which is preferably a polymerizeddifluorinated ethylene having the formula

    CH.sub.3 --CF.sub.2 --CH.sub.2 -CF.sub.2 --.sub.n CH.sub.2 --CHF.sub.2

where n is an integer.

In practice such polymers can be represented by the formula

    --CH.sub.2 --CF.sub.2).sub.m

where m is equal to n+2. The polyolefin can be of the low-density typewith a molecular weight of 7000 to 12000.

Surprisingly, the latter material has characteristics similar topolytetrafluoroethylene but is far more effective upon ultrasonicwelding in the manner described as is important to the instantinvention, than polytetrafluoroethylene.

According to the principles set forth in my earlier application Ser. No.885,184, the cover layers on both sides of the metal support arethermally welded together continuously along their edges around all ofthe edges of the metal support structure except that portion of thesupport structure which can form a terminal tab or tongue. This affordsa full or complete sheathing of the active mass.

It will be apparent from the foregoing that the polyolefin openworklayers applied to the support are fused together at points within theopenings of the support, constitute a pocketed structure adapted topermit volume change of the active mass while serving as a completelystable structure in the static sense. Mechanical stresses are taken upby the polyolefin layers applied directly to the metal support and thenresult in a loosening of the mass from the latter. The polyolefinopenwork layers themselves are connected through the support and hencecannot be dislodged, torn or otherwise released therefrom. In addition,the sheathing layers of glass fabrics or glass felt are bonded to theunderlying polyolefin layers over their entire surfaces and remainstructurally connected to the basic electrode components even under themost extreme of operating conditions.

According to the invention, moreover, the conductive metal support isconstituted as a grid, advantageously a cast grid, although preferablyit is provided with openings by the slitting/expansion technique knownin the art. In other words, the metal support may be a so-calledexpanded-metal structure.

It is also significant that the metal support is covered on both sideswith the polyolefin synthetic-resin layers which are formed fromsynthetic-resin strands fused together through the openings of the metalsupport. The integration of the polyolefin layers with one another andwith the metal structure through the openings of the latter by apoint-like fusion of the strands or filaments of the polyolefin layersprovides a multipoint connection.

The fabrication technique is relatively simple and requires only asuccessive or single-step welding of the polyolefin layers together bythe application of ultrasonic welding fields and the simultaneousapplication of pressure. A continuous welding operation can be carriedout by passing the electrode structure between a pair of ultrasonicallyenergized rollers. Preferably, however, the electrode structure ispressed between a pair of ultrasonically energized platens.

The active mass can be applied to the pockets thereby formed in thepolyolefin layers, preferably after a fine-porous sheathing or coverlayer is applied to one side of the electrode structure. After theactive mass is applied, the other side of the electrode can be coveredwith a sheathing layer of the fine-porous glass material and the entireassembly subjected to a hot pressing operation to bond the newly appliedsheathing layer to the previously applied screen, net, fabric or web ofpolyolefin material and to effect the bonding around at least threesides of the rectangular electrode structure.

The active mass can be applied by dry pressing, moist pasting or anyother conventional technique.

More specifically, the low-homologous polyolefins of the presentinvention are low-pressure polyethylene

    [--CH.sub.2 --CH.sub.2 --].sub.n

which, for the present purpose, should have a chain length n of 6×10³ to4×10⁶, or high-pressure polyethylene with a chain length n of 5×10³ to5×10⁴, or polypropylene ##STR1## of corresponding chain lengths n, orpolybutylenes of similar chain lengths.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a vertical cross-sectional view through an electrode accordingto the invention, the portions thereof being shown in greatly enlargedscale and in diagrammatic form;

FIG. 2 is an elevational view of the electrode, partly broken away;

FIG. 3 is a cross-sectional view illustrating features of the invention,shown also in diagrammatic form;

FIG. 4 is a view similar to FIG. 2 but illustrating another feature ofthe invention; and

FIG. 5 is a cross-sectional view through an apparatus for carrying out aprocess for making the electrode of FIGS. 1 through 4.

SPECIFIC DESCRIPTION

The composite electrodes shown in FIGS. 1 through 4 may be used forstorage batteries or electrical accumulators in accordance with theexamples given below. Basically, each electrode comprises a metalsupport or carrier 1, shown as a flat plate, provided with openings 3 ina surface distribution, these openings being throughgoing so that theyare accessible from either side of the support.

On both sides of the support, there are provided respective polyolefinor partially fluorinated polyolefin layers 2 formed from filaments orstrands and hence having an open configuration. These polyolefin layers2 can be termed polyolefin openworks. The polyolefin layers 2 formpockets to receive the active mass which has been shown at 7 in thedrawing. The active mass can be pasted in the pockets in a moist stateand can then be permitted to dry or can be applied under pressure as apowder.

The synthetic-resin layers 2 are welded together at point welds 4 byultrasonic energy and the application of pressure. The welds 4 are thusformed in the openings 3 of the metal support.

After the application of the active mass, the latter can be sheathedwith fine-porous cover layers 5 of glass-fiber material. The coverlayers 5 can be nonwoven fiber fleeces or mats (felts), the mats 5 beingfused at point 4' to the underlying layer 2.

As can be seen from FIG. 3, the metal support may be an expanded metalgrid in which the openings are formed by slitting the metal plate andthen stretching the same to rotate portions of the plate between theslits at an angle to the plane of the plate.

The fusion points between the mats 5 and the screens 2 of polyolefinfilaments have been shown at 4' in greater detail in FIG. 3.

FIG. 4 shows that the edges of the mats 5 can extend beyond the outlinesof the electrode so that they can be welded together and to anyprojecting portion of the grids or openworks 2 along these edges. Theprojecting edges are represented at 9 in FIG. 4 and the fusion seams at8 around the edges of the metal support. The result is a continuous weldseam around the active mass fully encapsulating the latter andrepresented at 6 in FIG. 1.

In FIG. 5 I have shown the fusion of the assembly of FIG. 4 togetherbetween a pair of platens 11 and 12 of an ultrasonic welding tool,pressure being applied in the direction of arrows 13 and 14 and theultrasonic energy being delivered by the ultrasonic welding source 10.The polyolefin is preferably polyethylene or difluoroethylene. The term"glass" is here used specifically although the less preferred "mineralwool" or "asbestos" can be used in each case.

SPECIFIC EXAMPLES

An expanded-metal plate of lead/calcium/tin alloy having dimensions, inthe region to be covered by the active mass, of 70 by 120 mm andprovided with openings which can each have an area of about 4 mm square,is used as the metal support. The metal plate has a thickness of about2.3 mm and is covered on opposite sides with synthetic-resin screens ofpolydifluoroethylene having a thickness of 0.5 mm and a spacing of thestrand of the screen of about 2 to 3 mm.

All three layers are ultrasonically welded together as illustrated inFIG. 5 with the application of a pressure of 10 to 30 c/cm² using a flatultrasonic welding tool. Investigation shows that the twosynthetic-resin grids are fused together at points with the openings ofthe metal support.

In a second, thermal welding operation, a nonwoven fiber mat of glassfibers is fused to one side of the electrode assembly, i.e. to one ofthe synthetic-resin grids. To this assembly is applied, by doctoring, amoist paste of a positive active mass constituted of a lead/lead dioxidemixture. Such mixtures are commonly used in the storage-battery industryfor lead-acid batteries. After drying, the open side of the assembly issheathed by a second fine-porous glass-fiber nonwoven mat welded to thesynthetic-resin screen on the formerly open side of the structure. Theactive mass is thus distributed in the many small pockets of thesynthetic-resin screens. The positive electrode is immersed in sulfuricacid and charged in the usual way.

EXAMPLE II

Using a conventional lead/antimony alloy grid for starting batterieshaving a thickness of 1.3 mm, the process described above is carried outwith synthetic-resin webs on either side of the grid. Thesynthetic-resin layers or sheets have a thickness of 0.5 mm perforatedwith large openings having dimensions of about 3×3 mm.

EXAMPLE IIa

Using the grid of Example II and the perforated sheets describedtherein, the sheets are applied to the metal grid and the active mass ispasted in the openings of these sheets. After drying of the active mass,the lead/synthetic-resin structure is subjected to high energyultrasonic fusion to bond the synthetic-resin sheets to one anotherthrough the openings in the grid. In a subsequent step, the two glassfiber cover layers 5 are applied and welded to the synthetic-resinsheets and to one another along the edges of the electrode. Otherwisethe electrode is formed up as described in Examples I and II.

EXAMPLE IIb

The procedure of Example II is followed except that the glass fibercover layers 5 are applied without previous welding of thesynthetic-resin perforated sheets together and the entire assembly isfused ultrasonically together in a single step. In Examples IIa and IIb,the electrodes are formed up in sulfuric acid as described in Example I.

The electrodes can be made by the method described in theabove-identified copending application Ser. No. 885,180 which is herebyincluded by reference in toto.

I claim:
 1. A composite electrode for storage batteries and the like,comprising:a metal support provided with a multiplicity of throughgoingopenings; respective polyolefin synthetic-resin layers constituted asopenwork sheets of screened fabric, net or grid structure flanking saidsupport on opposite sides thereof, said layers being directly fusedtogether the openings in said support and at points within saidopenings; an electrochemically active mass received in the openworks ofsaid layers; and respective fine-porous cover layers in the form ofinorganic fiber fabric or felt overlying and bonded to the respectivepolyolefin layers.
 2. The composite electrode defined in claim 1 whereinthe polyolefin layers are formed of low-homologous polyolefin orpartially fluorinated polyethylene.
 3. The composite electrode definedin claim 2 wherein the polyolefin layers are formed of polyethylene,polypropylene or polybutylene.
 4. The composite electrode defined inclaim 2 wherein the polyolefin layers are formed ofpolydifluoroethylene.
 5. The composite electrode defined in claim 2wherein said fine-porous cover layers are composed of glass, mineralwool or asbestos fibers.
 6. The composite electrode defined in claim 2wherein the polyolefin layers are welded together around edges of saidsupport.
 7. The composite electrode defined in claim 2 wherein thepolyolefin layers are screens of polyolefin strands.
 8. The compositeelectrode defined in claim 2 wherein the polyolefin layers are fabrics.9. The composite electrode defined in claim 2 wherein the polyolefinlayers are nets.
 10. The composite electrode defined in claim 2 whereinthe cover layers are nonwoven mats of glass fiber.